Thermal spray coated member and thermal spraying method therefor

ABSTRACT

It is an object to extend the service life of a thermal-sprayed coating thermal-sprayed onto a molten metal resistant member and to prevent adhesion of a molten metal. 
     In the molten metal resistant member, a contact portion coming into contact with a molten metal including Zn and/or Al has been covered with the thermal-sprayed coating. The thermal-sprayed coating is formed by using thermal-spraying oxide-based ceramic particles having an average particle diameter as a median diameter of 10 μm or smaller. The thermal-spraying particles are thermal-sprayed at a high flying particle velocity of 1,000 m/sec or higher with only the surfaces of the flying thermal-spraying particles being in a semi-molten state and the inside of the particles being in a solid state. The resistance to corrosion by a molten metal, insulating properties, the resistance to washing with acids, and the ability to prevent adhesion of the molten metal are thereby improved.

TECHNICAL FIELD

The present invention relates to a thermal spray coated member producedby thermally spraying thermal-spraying particles thereonto and to amethod for producing the thermal spray coated member.

BACKGROUND ART

In one known method for forming a coating on the surface of a steelplate, the steel plate is immersed in a bath containing a molten metalsuch as zinc, aluminum, or a zinc-aluminum alloy. Conveying rollers forconveying the steel plate are provided in the bath. However, the moltenmetal may permeate and erode the conveying rollers. One known measure toprevent the permeation and erosion is to coat the surfaces of theconveying rollers with a protective coating.

One known method for forming such a protective coating is a highvelocity gas spraying method. Patent Literatures 1 and 2 disclosemethods in which a WC—Co-based or WC—B—Co-based cermet material isthermal-sprayed using the high velocity gas spraying method. PatentLiterature 3 discloses a method in which a thermal-sprayed layer formedby plasma-spraying a ceramic such as chromia onto the coating issubjected to pore sealing. Patent Literature 4 discloses a method inwhich chromium carbide is formed on the surface of a thermal-sprayedlayer and in pores therein to seal the pores. Patent Literature 5discloses a method in which a composite ceramic composed ofSiO₂—Cr₂O₃—Al₂O₃ is thermal-sprayed onto the surface of a substrateusing a plasma gun or a gas spray gun to seal pores with chromium oxide.Patent Literature 6 discloses a method in which, in order to preventcorrosion in molten zinc caused by local cells formed from WC in athermal-sprayed cermet coating and Co therein serving as a binder, thecomponents of the binder are controlled so that the difference inimmersion potential is 80 mV or lower.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Laid-Open No. Sho.    48-11237-   Patent Literature 2: Japanese Patent No. 2553937-   Patent Literature 3: Japanese Patent Application Laid-Open No. Hei.    05-209259-   Patent Literature 4: Japanese Patent Application Laid-Open No. Hei.    08-109458-   Patent Literature 5: Japanese Patent Application Laid-Open No.    2002-4016-   Patent Literature 6: Japanese Patent Application Laid-Open No.    2009-19271

SUMMARY OF INVENTION Technical Problem

However, with thermal spraying methods and pore sealing treatmentdescribed in Patent Literatures 3 to 5, the bonding strength between theceramic particles is weak, and the porosity of the thermal-sprayedceramic layers is high, so that their structures are brittle. Inaddition, since the porosity is high, the insulating properties of thethermal-sprayed ceramic layers are low, so that a corrosion potentialcannot be prevented.

Even with the method described in Patent Literature 6, the corrosionpotential cannot be reduced to zero, so that the progress of dissolutionloss cannot be prevented.

Accordingly, it is an object of the present invention to provide athermal spray coated member including a thermal-sprayed oxide-basedceramic coating that includes thermal-spraying particles bonded withhigh bonding strength, is dense, resists deterioration, and has highinsulating properties.

Solution to Problem

To achieve the above object, the thermal spray coated member accordingto the present invention includes a thermal-sprayed oxide-based ceramiccoating formed using thermal-spraying oxide-based ceramic particleshaving an average particle diameter as a median diameter of 10 μm orsmaller. The thermal-sprayed oxide-based ceramic coating is dense,resists deterioration, and has high insulating properties. The aboveaverage particle diameter is a median diameter measured by a laserdiffraction scattering measurement method.

Advantageous Effects of Invention

The present invention can provide a thermal spray coated memberincluding a thermal-sprayed oxide-based ceramic coating that includesthermal-spraying particles bonded with high bonding strength, is dense,resists deterioration, and has high insulating properties.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an optical microscope photograph of a cross section of aconventional high-velocity-gas-sprayed WC—B—Co-based cermet coating thathas been immersed in a molten metal including 55% by weight of aluminumand 45% by weight of zinc at 873K for 16 days.

FIG. 2 is an optical microscope photograph of a coating in PatentLiterature 3 formed by thermal-spraying an oxide-based ceramic (chromia)onto a high-velocity-gas-sprayed WC—B—Co-based cermet coating and thensubjecting the resultant coating to pore sealing.

FIG. 3 is an optical microscope photograph of a cross section of a testpiece immersed in a molten metal including 55% by weight of aluminum and45% by weight of zinc at 873K for 16 days, the test piece including: athermal-sprayed cermet coating thermal-sprayed using a high velocity gasspraying apparatus; and a coating with unsealed pores that has beenformed thereon by thermal-spraying 6 μm fine thermal-spraying grayalumina particles to 50 μm using a high velocity gas spraying apparatusin a manner according to the present invention.

FIG. 4 is a cross-sectional view of a high velocity gas sprayingapparatus.

FIG. 5 is a cross-sectional view of a high velocity gas sprayingapparatus in Japanese Patent Application Laid-Open No. 2009-179846.

FIG. 6 is a schematic diagram of a blast abrasion tester.

DESCRIPTION OF EMBODIMENTS

Thermal spray coated members of the present invention include variousmembers that require to have a thermal-sprayed oxide-based ceramiccoating that includes thermal-spraying particles bonded with highbonding strength, is dense, resists deterioration, and has highinsulating properties. Examples of such members include members thatcome into contact with a molten metal, conveying rollers that conveyhigh-temperature glass with a temperature of 473K or higher, and hearthrolls disposed in a heat treatment furnace for steel plates.

Examples of the members that come into contact with a molten metalinclude: conveying rollers disposed inside a storage container storing amolten metal for plating steel plates to convey the steel plates;conveying rollers disposed outside the storage container to convey steelplates with the molten metal adhering thereto; molds into which a moltenmetal is poured; dippers used for the molds; and feed pumps for feedinga molten metal.

Embodiments of the present invention will next be described in detail,and a member that comes into contact with a molten metal is used as anexample.

As described in Patent Literatures 4 and 6, examples of the propertiesrequired for a thermal-sprayed coating thermal-sprayed onto a moltenmetal resistant member such as a conveying roller used for molten zincplating include: (1) erosion by molten zinc is less likely to occur; (2)the thermal-sprayed coating resists abrasion even when it comes intocontact with a strip (steel plate); (3) adhering molten zinc can beeasily removed, and maintenance is easy; (4) the thermal-sprayed coatingused as a plating member has long life and is low cost; (5) thethermal-sprayed coating can easily resist thermal shock when it isimmersed in high-temperature molten zinc; and (6) the corrosionpotentials of the components of a thermal-sprayed cermet coating inmolten zinc are low.

However, in the thermal-sprayed coatings in Patent Literatures 1 and 2produced by thermal-spraying a WC—Co-based or WC—B—Co-based cermetmaterial by the high velocity gas spraying method, the Co binder iseroded by molten zinc or aluminum, and the molten metal permeates anderodes the thermal-sprayed coatings through through-pores and pores.This causes a corrosion potential to occur, and the thermal-sprayedcoating flakes off and wears. The molten metal permeating thethermal-sprayed coating is embedded in the thermal-sprayed coatings andcannot be easily removed, causing a reduction in plating quality of thesteel plate.

The coating described in any of Patent Literatures 3 to 5 produced bythermal-spraying an oxide-based ceramic onto a thermal-sprayed cermetcoating easily cracks and flakes off by thermal shock and has lowcoating strength and wear resistance. The thermal-spraying ceramicparticles are bonded to each other by subjecting the coating to poresealing. However, since the bonding structure is brittle, cracks andexfoliation easily occur when external mechanical force or thermalstress is applied to the coating, and the insulating properties thereofare low. The thermal-sprayed ceramic coating has a pumice-like structurecontaining pores and through pores in an amount of about 15% to about25%. Therefore, a molten metal easily permeates the pores and throughpores in the sprayed ceramic coating, and the molten metal and the oxidethereof (dross) are embedded in the thermal-sprayed coating and cannotbe easily removed, causing a reduction in plating quality.

In recent years, in the context of reducing cost, improving productionefficiency, etc., there is a need for a further improvement in theservice life of conveying rollers. In addition, since requirements forthe quality of plated steel plates are becoming more strict, there is astrong need for a molten metal and oxides thereof (dross) adhering tothe conveying rollers to be easily removed, in order to take measuresagainst plating quality defects caused by the molten metal and oxidesthereof.

Since the conveying rollers are reused after zinc and aluminum adheringthereto are dissolved by washing with an acid such as sulfuric acid orhydrochloric acid, the conveying rollers are required to have a longlife against corrosion by acids.

The present inventor has analyzed a conventional thermal-sprayed coatingto analyze the cause of the exfoliation and wear of the thermal-sprayedcoating. FIG. 1 is an optical microscope photograph of a cross sectionof a conventional high velocity gas sprayed WC—B—Co-based cermet coatingthat has been immersed in a molten metal including 55 percent by weightof aluminum and 45 percent by weight of zinc at 873K for 16 days. Zincand aluminum being molten metal components permeated the thermal-sprayedcoating, and the thermal-sprayed coating was cracked and nearly flakedoff. The molten metal eroded and permeated Co used as a binder of thethermal-sprayed coating and also caused permeation and erosion throughthrough-pores and pores.

FIG. 2 is an optical microscope photograph of a cross section of acoating in Patent Literature 3 formed by thermal-spraying an oxide-basedceramic (chromia) onto a high-velocity-gas-sprayed WC—B—Co-based cermetcoating. Generally, a ceramic has a high melting point and is thereforethermal-sprayed using a plasma gun. Since the temperature of the plasmafrom the plasma gun is about 30,000K, the thermal-spraying particles arecompletely melted and impinge on a substrate as molten droplets.However, since the velocity of the thermal-spraying particles is as lowas about 250 m/sec, the thermal-sprayed coating contains pores in anamount of 15% to 25%. The bonding strength between the thermal-sprayingparticles is, however, low, and the thermal-spraying particles arebonded through a pore sealing material. However, not all the pores canbe filled. Although the oxide ceramic itself does not react with themolten metal, the molten metal is embedded in the pores, and thisprevents the molten metal from being removed. The pore sealing materialis cracked and flakes off when thermal stress or an external mechanicalforce caused by contact with a steel plate is applied to the coating,and the molten metal is further embedded in the cracks and exfoliation.This makes the molten metal more difficult to remove.

The plasma sprayed oxide-based ceramic particles are quenched from amolten droplet state. Therefore, a large number of micro-cracks arepresent, and this weakens mechanical strength and causes a reduction ininsulating properties.

The present inventor has found that a long-service life molten metalresistant member that allows zinc to be easily removed, can preventoccurrence of a corrosion potential, can prevent permeation of acids soas to resist repeated washing with the acids, and has wear resistanceand resistance to corrosion by a molten metal can be produced if athermal-sprayed oxide-based ceramic coating that includes particlesbonded with high bonding strength, is dense, resists deterioration, andhas high insulating properties can be obtained.

A thermal-sprayed coating used for a molten metal resistant memberundergoes strong thermal shock when the thermal-sprayed coating comesinto contact with a molten metal. The toughness of a thermal-sprayedceramic material is lower than that of thermal-sprayed metal and cermetmaterials. Therefore, in a conventional method, the amount of pores inthe thermal-sprayed ceramic material is increased to form a pumice-likemicrostructure, and thermal stress is relaxed by thermal deformation ofthe microstructure. Since the temperature of the plasma from a plasmagun is about 30,000K, the thermal-spraying particles are completelymelted and impinge on a substrate as molten droplets. However, since thevelocity of the thermal-spraying particles is as low as about 250 m/sec,the thermal-sprayed coating inevitably contains pores in an amount of15% to 25%. When the ceramic is once melted and thermal-sprayed, itundergoes a phase change and is transformed, and the ceramic after thephase change is similar to but different from the original ceramic inbulk form.

The present inventor has found a high thermal shock resistant thermalspray coated member including a thermal-sprayed coating formed bythermally spraying thermal-spraying oxide-based ceramic particles havingan average particle diameter as a median diameter of 10 μm or smaller.More specifically, the thermal spray coated member comes into contactwith a molten metal, and each of the thermal-spraying particles in thethermal-sprayed coating includes a surface layer portion that has beenonce thermally melted and then solidified and an inner layer portionthat has not been thermally melted during thermal spraying.

FIG. 4 is a cross-sectional view of a high velocity gas sprayingapparatus. The thermal-sprayed coating is thermal-sprayed from the highvelocity gas spraying apparatus shown in FIG. 4. Kerosene used as fuelis fed inside a combustion chamber 2 and combusted with high-pressureoxygen, and the pressure inside the combustion chamber becomes as highas about 0.7 MPa. The combustion gas is converted to high-temperaturesupersonic gas with a velocity of about 3,270K and 3,000 m/sec through aLaval nozzle 3 disposed at the outlet of the combustion chamber.Thermally spraying particles sprayed from a thermal-spraying particlespraying nozzle 4 are supplied to the high-temperature supersonic gas.The supplied thermal-spraying particles are heated, accelerated, andthen sprayed onto an member to be spray-coated.

The configuration of the high velocity gas spraying apparatus shown inFIG. 4 will next be described in detail. The high velocity gas sprayingapparatus includes three main components, i.e., a combustion chambertail plug 1 disposed on its rear end, the combustion chamber 2 disposedin front of the plug 1 in the direction of spraying, and a spray nozzle5 connected to the combustion chamber 2.

A fuel supply port 7 for supplying fuel such as kerosene at highvelocity in the forward direction of spraying and an oxygen supply port8 for supplying oxygen gas at high velocity in the forward direction ofspraying are disposed in the combustion chamber tail plug 1.

The combustion chamber 2 to which the combustion chamber tail plug 1 isattached is formed into a cylindrical shape, and the Laval nozzle 3having a shape in which its diameter tapers and then gradually increasesis formed at the connection portion between the combustion chamber 2 andthe spray nozzle 5.

The spray nozzle 5 connected to the combustion chamber 2 through theLaval nozzle 3 is a copper tube having an inner diameter of about 11 mmand a length of about 10 cm to about 20 cm and is cooled by water fromthe outside. A thermal spraying material supply section 4 for supplyingthe thermal spraying material is disposed in the spray nozzle 5 andlocated on a side close to the Laval nozzle 3. A material prepared byadding a material suitable for the required characteristics such as wearresistance to a Ni-, Ni—Cr-, or Co-based alloy is used as the thermalspraying material.

When thermal spraying is performed, first, the fuel and oxygen suppliedfrom the kerosene supply port 7 and from the oxygen supply port 8disposed in the combustion chamber tail plug 1 are combusted in thecombustion chamber 2. During combustion, the combustion gas in thecombustion chamber 2 has a pressure of about 0.7 MPa and a combustiontemperature of about 3,000° C. The combustion gas is fed to the Lavalnozzle 3, accelerated to a sonic to supersonic speed when the gas passesthrough the Laval nozzle 3, and then supplied to the spray nozzle 5. Thethermal spraying material is sprayed from the thermal spraying materialsupply section 4 into the accelerated combustion gas at the connectionportion between the Laval nozzle 3 and the spray nozzle 5. The thermalspraying material is accelerated and heated by the combustion gas. Theflow of the combustion gas and the thermal spraying material issmoothened when they pass through the spray nozzle 5, and the resultantflow with improved convergence properties is sprayed from the tip of thespray nozzle 5. The thermal spraying material is thereby sprayed at veryhigh velocity and can be thermal-sprayed onto an member to bespray-coated.

White alumina having an average particle diameter as a median diameterof 40 μm, which is an oxide-based ceramic commonly used in the highvelocity gas spraying apparatus in FIG. 4, was sprayed into the sprayingapparatus to perform a thermal spray test. However, no film wasdeposited.

Therefore, an Accuraspray thermal spray measuring apparatus,manufactured by Sulzer Metco, was used to perform measurement. Thetemperature of the surfaces of the flying thermal-spraying particles wasfound to be 2,053K, and the velocity of the thermal-spraying particleswas found to be 815 m/sec. However, the melting point of the whitealumina is 2,302K, and therefore a film was not deposited because thetemperature and velocity were insufficient.

Assuming that the thermal-spraying particles are perfect spheres. Thentheir surface area is proportional to the square of the diameter, andthe volume is proportional to the cube of the diameter. Therefore, thespecific surface area increases inversely proportional to the diameter.For example, when the diameter of the thermal-spraying particles isreduced from 40 μm to 4 μm, the specific surface area increases by afactor of 10. The weight becomes 1/1,000. The thermal-spraying particlessprayed into the combustion gas are heated from their surfaces andsprayed through the flow of the combustion gas. Therefore, when finerthermal-spraying particles are used, it is expected that the temperatureand velocity of the thermal-spraying particles increase.

White alumina particles having an average particle diameter as a mediandiameter of 4 μm were sprayed, and measurement was performed using theAccuraspray thermal spray measuring apparatus, manufactured by SulzerMetco. The surface temperature of the flying thermal-spraying particleswas found to be 2,700K, and the velocity of the thermal-sprayingparticles was found to be 2,750 m/sec. This surface temperature ishigher than the melting point of the white alumina being 2,302K. Thewhite alumina particles were actually thermal-sprayed, and a 6 μm filmwas deposited in one pass. The coating was subjected to X-raydiffraction. It was found that an α phase which is the phase of thewhite alumina in bulk form remained unchanged and that a γ phasegenerated by fusion was present only in very limited regions. Therefore,the physical properties of the coating were very close to those of thewhite alumina in bulk form.

White alumina particles having an average particle diameter as a mediandiameter of 10 μm were sprayed, and measurement was performed using theAccuraspray thermal spray measuring apparatus manufactured by SulzerMetco. The surface temperature of the flying thermal-spraying particleswas found to be 2,400K, and the velocity of the thermal-sprayingparticles was found to be 1,000 m/sec. This surface temperature ishigher than the melting point of the white alumina being 2,302K. Thewhite alumina particles were actually sprayed, and a 1 μm film wasdeposited in one pass.

It was found that the formation of a film of an oxide-based ceramic bythermal spraying, which had been considered impossible, could beachieved by using the high velocity gas spraying apparatus. Morespecifically, this can be achieved by using thermal-spraying oxide-basedceramic particles having an average particle diameter as a mediandiameter of 10 μm or smaller. In addition, the thermal-sprayingparticles are thermal-sprayed at a high flying particle velocity of1,000 m/sec or higher such that only the surfaces of the flyingthermal-spraying particles are semi-melted while the inside of theparticles is in a solid state.

With the conventional high velocity gas spraying apparatus, thermalspraying could be performed for a short time. However, the inner surfaceof the nozzle 5 wore, and molten thermal-spraying particles adhered tothe inner surface of the nozzle 5 and then flaked off, causing spraydefects called spitting.

FIG. 5 is an example of a high velocity gas spraying apparatus disclosedin Japanese Patent Application Laid-Open No. 2009-179846 by the presentinventor. The inner diameter of an inner tube 2 which is disposed at oneend of the high velocity gas spraying apparatus 1 is the same as theinner diameter of a nozzle of the high velocity gas spraying apparatus.Air, nitrogen gas, or the like injected from a gas injection port 4 isinjected through a cylindrical gap between the inner tube 2 and an outertube 3 and forms a cylindrical gas tunnel to prevent oxidation of thethermal-spraying particles. When an oxide-based ceramic is sprayed, thegas tunnel may not be used. The fine powdery thermal-spraying particlesare sprayed at one end from a thermal spraying powder spraying port 5into the inside and center of high-temperature and high-velocitycombustion gas 6 and then converted to high temperature and highvelocity particles.

An oxide-based ceramic was thermal-sprayed using the high velocity gasspraying apparatus in FIG. 5. No abrasion of the nozzle and no spittingoccurred, and the oxide-based ceramic could be thermal-sprayed stably.

The thermal-spraying oxide-based ceramic particles that are in the formof fine powder having an average particle diameter as a median diameterof 10 μm or smaller and have been sprayed into the high velocity gasspraying apparatus are heated such that their surface temperature isclose to the temperature of the combustion gas in the high velocity gasspraying apparatus and are accelerated to a velocity close to thevelocity of the combustion gas. However, the oxide-based ceramic has lowthermal conductivity, and the distance between the spraying apparatusand the member to be spray-coated is short (200 mm in this testexample). The heating time is therefore short, and the flyingthermal-spraying oxide-based ceramic particles impinge on the member tobe spray-coated at a supersonic velocity of 1,000 m/sec of higher, withonly the surfaces of the particles being in a semi-melted state and theinside being in a solid state. Therefore, in contrast to athermal-sprayed oxide-based ceramic coating formed using a conventionalplasma gun, a thermal-sprayed coating that is dense, includesthermal-spraying particles bonded with high bonding strength, stronglyresists deterioration, and has insulating properties can be obtained.Since high compressive residual stress remains present in thethermal-sprayed coating, high bonding strength between the particles isobtained, and resistance to cracks and exfoliation caused by externalmechanical force and thermal shock is obtained. More specifically, eachof the thermal-spraying particles impinging on the member to bespray-coated includes a surface layer portion that has been oncethermally melted and then solidified and an inner layer portion that hasnot been thermally melted and is in a state before the thermal-sprayingparticles are fed to the high velocity gas spraying apparatus. Thesurface layer portions of adjacent thermal-spraying particles are firmlybonded with no gaps, and a thermal-sprayed coating having theabove-described properties can thereby be obtained. In the conventionaltechnology, pore sealing treatment is necessary. Thermal spraying takesone day for, for example, a roller to be used in a zinc bath, and poresealing and heat treatment takes 4 days. In the present invention, poresealing is not necessary. This is highly effective for the cost and thetime for completion.

FIG. 3 is an optical microscope photograph of a cross section of a testpiece immersed in a molten metal including 55% by weight of aluminum and45% by weight of zinc at 873K for 16 days, the test piece including: athermal-sprayed Mo—Co, Cr—B-based cermet coating thermal-sprayed usingthe high velocity gas spraying apparatus; and a coating with unsealedpores that has been formed thereon by thermal-spraying 6 μm finethermal-spraying gray alumina particles to 50 μm using the high velocitygas spraying apparatus. The thermal-sprayed fine powdery gray aluminacoating layer was dense, and no adhering molten metal was found. Thefine powdery gray alumina layer on the surface protected thethermal-sprayed cermet coating therebelow, and the thermal-sprayedcermet coating was completely sound. The test piece was immersed insulfuric acid for 12 hours to perform an acid resistance test. The finepowdery gray alumina coating in the surface layer and thethermal-sprayed cermet coating therebelow were completely sound, and thetest piece exhibited good properties as a molten metal resistant member.Therefore, it was found that even a thermal-sprayed oxide-based ceramiccoating that has molten metal resistance and undergoes thermal shock canbe practically used when the thermal-sprayed oxide-based ceramic coatingis formed not to have a pumice-like porous structure, is dense, includesthermal-spraying particles bonded with high bonding strength, resistsdeterioration, and has high insulating properties.

Embodiments of the present invention based on the above findings willnext be described in detail.

Embodiment 1

(1) A thermal spray coated member having high thermal shock resistance,the thermal spray coated member comprising a thermal-sprayed coatingformed by thermally spraying thermal-spraying oxide-based ceramicparticles having an average particle diameter as a median diameter of 10μm or smaller, wherein each of the thermal-spraying particles in thethermal-sprayed coating includes a surface layer portion that has beenthermally melted and then solidified and an inner layer portion that hasnot been thermally melted during thermal spraying.

The oxide-based ceramic is an oxide, is therefore stable, and has goodfeatures such as wear resistance, corrosion resistance at hightemperature, resistance to corrosion by acids, heat insulatingproperties, and electrical insulating properties. Therefore, such anoxide-based ceramic can be used for a long time for a molten metalresistant member that comes into contact with a molten metal includingZn and/or Al and is not used in a reducing atmosphere. In the presentinvention, the molten metal including Zn and/or Al is not a limitation,and high-temperature glass with a temperature of 475K or higher can alsobe used.

Embodiment 2

(2) In the configuration in (1), the thickness of the oxide-basedceramic coating is preferably 50 μm or smaller.

Conventional thermal-spraying particles have an average particlediameter as a median diameter of about 40 μm. Therefore, to preventthrough-pores that reach a substrate, the thickness must be at least 5times the minimum diameter of the particles, i.e., 200 μm. However, thecoating of the present invention that includes particles with an averageparticle diameter as a median diameter or 10 μm or smaller, is dense,and has high insulating properties can exert its performance even whenthe sprayed thickness is 50 μm or smaller, and a significant reductionin cost can be achieved.

Embodiment 3

(3) In the configuration in (1) or (2), the thermal spray coated memberpreferably further comprises a thermal-sprayed undercoating, thethermal-sprayed undercoating being a cermet- or metal-basedthermal-sprayed coating that has been thermal-sprayed as a primercoating for the thermal-sprayed oxide-based ceramic coating, and thethickness of the thermal-sprayed undercoating is preferably set to 200μm or smaller.

TABLE 1 shows the thermal expansion coefficients of various materials.Generally, in a molten zinc bath, when the difference in thermalexpansion coefficient is less than about 60%, the difference causes nocracks. However, when the difference in thermal expansion coefficient islarger than 60%, cracks are more likely to occur. Generally, theoxide-based ceramic used for the upper layer has a small thermalexpansion coefficient, and stainless steel used for a substrate has alarge thermal expansion coefficient. Therefore, by providing, betweenthe substrate and the upper layer, a thermal-sprayed undercoating havingan intermediate thermal expansion coefficient between those of thesubstrate and the upper layer, the occurrence of cracks due to thedifference in thermal expansion coefficient can be prevented.

When gray alumina is thermal-sprayed and SUS316L being austenitestainless steel is used for the substrate, it is preferable that ametal-based material such as STELITE#6 and WC—B—Co-based cermet bethermal-sprayed to form an undercoating.

In the configuration in (1), gray alumina or white alumina may be usedfor the thermal-spraying particles. In this case, the thermal-sprayingparticles in the thermal-sprayed coating are such that the crystalstructure of the surface layer portion has been changed from the α phaseto the γ phase by heat fusion and the crystal structure of the innerlayer portion has not been changed and is the α phase.

TABLE 1 THERMAL EXPANSION MATERIAL COEFFICIENT NOTE UPPER GRAY  7.4 ×10⁻⁶/K OXIDE-BASED LAYER ALUMINA CERAMIC WHITE  8.0 × 10⁻⁶/K OXIDE-BASEDALUMINA CERAMIC LOWER WC-B—Co  9.3 × 10⁻⁶/K CERMET-BASED LAYER BASEDMo—Co, Cr—B  9.2 × 10⁻⁶/K CERMET-BASED BASED STELITE#6 15.0 × 10⁻⁶/KMETAL BASED SUB- SUS316L 19.3 × 10⁻⁶/K AUSTENITE-BASED SUS STRATE SUS41011.7 × 10⁻⁶/K MARTENSITE-BASED SUS SUS430 11.9 × 10⁻⁶/K FERRITE-BASEDSUS

Various oxide-based ceramics are listed in TABLE 3. Of these, grayalumina is produced by melting and reducing natural bauxite directly inan arc type electric furnace and is therefore inexpensive. In addition,the gray alumina has a low melting point, can be easily thermal-sprayed,and has high shock resistance, wear resistance, molten metal resistance,acid resistance, and molten metal releasability. When the gray aluminawas immersed in a molten metal including 55% by weight of aluminum/45%by weight of zinc at 873K for 16 days, the gray alumina was not damaged,as shown in the optical microscope photograph in FIG. 3.

A test piece 1 was produced by thermal-spraying conventional grayalumina having an average particle diameter as a median diameter ofabout 25 μm using a plasma gun, and a test piece 2 according to thepresent invention was produced by thermal-spraying gray alumina havingan average particle diameter as a median diameter of about 6 μm usingthe high velocity gas spraying apparatus in FIG. 5. The bondingstrengths between the thermal-spraying particles in the thermal-sprayedcoatings in the test pieces 1 and 2 were compared using a blast abrasiontester shown in FIG. 6. More specifically, 1 kg of a #70 aluminablasting material was sprayed onto each thermal-sprayed coating from adistance of 65 mm at an angle of 60°. A reduction in weight of the eachsprayed coating was measured, and the results for the test pieces werecompared. By causing the blasting material to impinge on thethermal-sprayed coating at an angle of 60°, thermal-spraying particlesin the thermal-sprayed coating are separated from each other, and theweight of the thermal-sprayed coating decreases. By comparing theamounts of the reduction in weight, the bonding strengths between theparticles can be compared. As shown in TABLE 2, the bonding strengthbetween the particles in the test piece 2 according to the presentinvention was found to be higher by a factor of 5.6 than that in thetest piece 1 including the conventional thermal-sprayed coating. Thethermal-sprayed coating in the test piece 1 was brittle. This may bebecause, since the thermal-sprayed coating in the test piece 1 containspores in an amount of 21%, the particles are not physically bonded. Inaddition, since the thermal-spraying particles have been completelymelted in plasma at about 30,000K, the phase change may also contributeto the brittleness.

TABLE 2 TEST PIECE 1 CONVENTIONAL TEST PIECE 2 THERMAL-SPRAYED PRESENTITEM COATING INVENTION THERMAL SPRAYING PLASMA GUN HIGH VELOCITYAPPARATUS Sulzer Metco GAS SPRAYING 10 MB Gun APPARATUS FIG. 5THERMAL-SPRAYED  25 μm  6 μm PARTICLE DIAMETER THICKNESS OF 200 μm 200μm THERMAL-SPRAYED FILM POROSITY 21% 0.1% WEIGHT REDUCTION 5.6 1(REFERENCE) RATIO INTERPARTICLE 1 (REFERENCE) 5.6 BONDING STRENGTH RATIO

TABLE 3 MAIN LINEAR MELT- COM- EXPANSION ING PONENT COEFFICIENT POINTTYPE (%) 10-6/K (K) ALUMINA GRAY Al₂O₃: Bal 7.4 2128 ALUMINA TiO₂: 2.5SiO₂: 1.0 WHITE Al₂O₃: +98 8.0 2272 ALUMINA ALUMINA- 15% TITANIA Al₂O₃:Bal 5.3 2113 TITANIA TiO₂: 15 40% TITANIA Al₂O₃: Bal 7.5 2113 TiO₂: 40ALUMINA-ZIRCONIA Al₂O₃: Bal 6.3 2143 ZrO₂: 24 ZIRCONIA  8% YTTRIA ZrO₂:Bal 9.7 2973 Y₂O₃: 8 25% YTTRIA ZrO₂: Bal 8.7 2873 MgO: 24 ZIRCON ZrO₂:Bal 7.6 2048 SiO₂: 33 ALUMINA-CHROMIA Al₂O₃: 50 8.0 2403 Cr₂O₃: BalCHROMIA Cr₂O₃: 9.6 2573 MULLITE 3Al₂O₃•2SiO₂ 5.6 2163

1. A thermal spray coated member having high thermal shock resistance,the thermal spray coated member comprising a thermal-sprayed coatingproduced by thermally spraying thermal-spraying particles of anoxide-based ceramic that have an average particle diameter of 10 μm orsmaller, the average particle diameter being a median diameter, whereineach of the thermal-spraying particles in the thermal-sprayed coatingincludes a surface layer portion that has been once thermally melted andthen solidified and an inner layer portion that has not been thermallymelted during thermal spraying.
 2. The thermal spray coated memberaccording to claim 1, wherein a thickness of the thermal-sprayedoxide-based ceramic coating is 50 μm or smaller.
 3. The thermal spraycoated member according to claim 1, further comprising a thermal-sprayedundercoating, the thermal-sprayed undercoating being a cermet- ormetal-based thermal-sprayed coating that has been thermal-sprayed as aprimer coating for the thermal-sprayed oxide-based ceramic coating, andwherein a thickness of the sprayed undercoating is 200 μm or smaller. 4.The thermal spray coated member according to claim 1, wherein theoxide-based ceramic is gray alumina.
 5. The thermal spray coated memberaccording to claim 1, wherein the thermal spray coated member is amolten metal resistant member produced by coating a contact portionthereof with the thermal-sprayed coating, the contact portion cominginto contact with a molten metal including Zn and/or Al orhigh-temperature glass with a temperature of 473K or higher.
 6. Thethermal spray coated member according to claim 1, wherein thethermal-spraying particles are grey alumina or white alumina particles,and the surface layer portion has a crystal structure of a γ phasechanged from an α phase by heat fusion, and the inner layer portion hasa crystal structure of the α phase.
 7. The thermal spray coated memberaccording to claim 2, further comprising a thermal-sprayed undercoating,the thermal-sprayed undercoating being a cermet- or metal-basedthermal-sprayed coating that has been thermal-sprayed as a primercoating for the thermal-sprayed oxide-based ceramic coating, and whereina thickness of the sprayed undercoating is 200 μm or smaller.
 8. Thethermal spray coated member according to claim 2, wherein theoxide-based ceramic is gray alumina.
 9. The thermal spray coated memberaccording to claim 3, wherein the oxide-based ceramic is gray alumina.10. The thermal spray coated member according to claim 7, wherein theoxide-based ceramic is gray alumina.
 11. The thermal spray coated memberaccording to claim 2, wherein the thermal spray coated member is amolten metal resistant member produced by coating a contact portionthereof with the thermal-sprayed coating, the contact portion cominginto contact with a molten metal including Zn and/or Al orhigh-temperature glass with a temperature of 473K or higher.
 12. Thethermal spray coated member according to claim 3, wherein the thermalspray coated member is a molten metal resistant member produced bycoating a contact portion thereof with the thermal-sprayed coating, thecontact portion coming into contact with a molten metal including Znand/or Al or high-temperature glass with a temperature of 473K orhigher.
 13. The thermal spray coated member according to claim 7,wherein the thermal spray coated member is a molten metal resistantmember produced by coating a contact portion thereof with thethermal-sprayed coating, the contact portion coming into contact with amolten metal including Zn and/or Al or high-temperature glass with atemperature of 473K or higher.
 14. The thermal spray coated memberaccording to claim 4, wherein the thermal spray coated member is amolten metal resistant member produced by coating a contact portionthereof with the thermal-sprayed coating, the contact portion cominginto contact with a molten metal including Zn and/or Al orhigh-temperature glass with a temperature of 473K or higher.
 15. Thethermal spray coated member according to claim 8, wherein the thermalspray coated member is a molten metal resistant member produced bycoating a contact portion thereof with the thermal-sprayed coating, thecontact portion coming into contact with a molten metal including Znand/or Al or high-temperature glass with a temperature of 473K orhigher.
 16. The thermal spray coated member according to claim 9,wherein the thermal spray coated member is a molten metal resistantmember produced by coating a contact portion thereof with thethermal-sprayed coating, the contact portion coming into contact with amolten metal including Zn and/or Al or high-temperature glass with atemperature of 473K or higher.
 17. The thermal spray coated memberaccording to claim 10, wherein the thermal spray coated member is amolten metal resistant member produced by coating a contact portionthereof with the thermal-sprayed coating, the contact portion cominginto contact with a molten metal including Zn and/or Al orhigh-temperature glass with a temperature of 473K or higher.